The present invention relates to an optical recording medium and an optical recording and reproduction apparatus that use a near-field light so as to realize a recording density which is beyond the diffraction limit of the light.
Recently, a variety of systems for carrying out a high-density recording by using the near-field light have been developed. For example, Japanese Journal Applied Physics, vol.39(2000), Part 1, No. 2B, pp.980-981, Part 1, No. 2B, February 2000, discloses readout method and writing method for an optical memory by use of the near-field light. The following description deals with such an art with reference to FIG. 4.
A light beam (laser beam) 101 converged by an object lens (not shown) is directed (projected) to an optical disk 102. The optical disk 102 is arranged so that a protection layer 112, a mask layer 113, a protection layer 114, a recording layer 115, and a protection layer 116 are provided on a disk substrate 111 in this order. Each thickness of the respective layers are as follows. More specifically, the disk substrate 111 has a thickness of 0.6 mm, the protection layer 112 has a thickness of 170 nm, the mask layer 113 has a thickness of 15 nm, the protection layer 114 has a thickness of 40 nm, the recording layer 115 has a thickness of 15 nm, and the protection layer 116 has a thickness of 20 nm.
Ge2Sb2Te5, that is a material of phase transition type, is used as the recording layer 115. A silver oxide layer is used as the mask layer 113. The light beam 115 causes the mask layer 113 to have a temperature distribution 117 shown in FIG. 1.
Silver oxide decomposes and the silver is deposited around a center portion of the light beam spot whose temperature is beyond a threshold temperature 118 in the temperature distribution 117. The deposition of silver causes each index of refraction in such a portion to change so as to provide in the mask layer 113 a scatterer 103 whose diameter is smaller than that of the light beam spot. This allows to generate a near-field light 105 around the scatterer 103. The near-field light 105 interacts with a mark 104 that has been recorded in the recording layer 115 so as to generate a propagation light. One part of the propagation light is readout as the reflection light. The protection layer 114 is set so that its thickness is equal to a distance that is not more than a distance which allows the near-field light 105 derived from the scatterer 103 to reach the recording layer 115. This allows to record or reproduce a record mark of not more than 100 nm.
However, the foregoing conventional art has the following problem. More specifically, the distance between the mask layer 113 and the recording layer 115 is short so as to cause the phase transition due to the thermal interference. This causes the recorded mark to be erased.
More specifically, in order for the mask layer 113 to have the scatterer 103, it is necessary to raise the temperature of the mask layer 113 to be not less than the threshold temperature 118. However, the distance between the mask layer 113 and the recording layer 115 is short so as to cause the heat of the mask layer 113 to be conducted to the recording layer 115 with ease. This allows the parts other than the recorded mark 104 to have the phase transition and the crystallization, thereby raising the problem that the recorded signal is gradually erased.
In view of the foregoing problem, the present invention is made, and its object is to provide an optical recording medium and an optical recording and reproduction apparatus that can avoid the erasure of the recorded mark and that can carry out the reproduction again and again.
In order to achieve the foregoing object, an optical recording medium in accordance with the present invention has a mask layer that changes in its index of refraction at a temperature of not less than a threshold temperature, and a recording layer that is provided away from the mask layer by a distance that is not more than a distance which allows a near-field light to reach, the recording layer being a magnetic layer.
With the optical recording medium, since the recording layer is a magnetic layer, information is magnetically recorded. Accordingly, even when the distance between the mask layer and the recording layer is short so as to cause the thermal interference, the information that has been recorded is not affected. Namely, the information that has been recorded is not changed as long as a magnetic field is not applied thereto. This ensures to avoid the conventional problem that the information that has been recorded is gradually erased.
The reproduction of the information is carried out as follows. More specifically, when the mask layer has a temperature rise, the temperature rise portion whose temperature is not less than the threshold temperature changes in its index of refraction so as to generate the near-field light around the portion in which the index of refraction is changed. Since the recording layer is provided away from the mask layer by the distance that is not more than a distance which allows the near-field light to reach, the near-field light interacts with the recording layer and is scattered. The scattered light (propagation light) is partially reflected so as to generate the reflected light. The information is reproduced in accordance with the reflected light.
Note that when the temperature of the temperature rise portion becomes lower than the threshold temperature, the portion whose index of refraction has changed returns to the previous index of refraction (the original index of refraction). In response to the moving of the temperature rise portion in the mask layer, the portion whose index of refraction changes is moved. This allows to carry out the reproduction again and again by use of the near-field light.
It is preferable that the magnetic layer is a magneto-optical recording layer. In this case, the recording of the information is carried out as follows. More specifically, the temperature of a recorded portion is raised so that the coercive force of the magneto-optical recording layer becomes substantially zero. Then, an external magnetic field is applied so as to reverse the direction of the magnetization of the recorded portion. Thus, the recording of the information is carried out.
During the reproduction of the information, the information that has been recorded in the magneto-optical recording layer is not changed as long as the external magnetic field is not applied thereto, even when the distance between the mask layer and the recording layer is short so as to cause the thermal interference therebetween. Namely, even when the temperature rise occurs in the recording layer during the reproduction, there is no reverse of magnetization in the recorded portion and the portions other than the recorded portion as long as the external magnetic field is not applied. This ensures to avoid that the recorded information is gradually erased.
In order to achieve the foregoing object, an optical recording and reproduction apparatus in accordance with the present invention uses the above optical recording medium and has (a) temperature rise means for raising the temperature of the optical recording medium, (b) temperature control means for controlling the temperature rise means so that the temperature rise during the reproduction is lower than that during the recording and so that the recording is not carried out with respect to the recording layer, (c) magnetic field generating means for generating a recording magnetic field that varies depending on the information to be recorded, and for applying the recording magnetic field to the optical recording medium, and (d) reproduction means for detecting a polarized component of light that has reflected from or transmitted through the optical recording medium so as to reproduce the information.
With the optical recording and reproduction apparatus, the magnetic field from the magnetic field generating means is applied to a target portion of the magnetic layer, thereby carrying out the recording. Meanwhile, during the reproduction, the temperature rise means is controlled by the temperature control means so that the temperature rise during the reproduction is lower than that of the recording. During this, the magnetization of the recording layer is not affected by the temperature rise. The temperature rise causes the index of refraction of the portion whose temperature is not less than the threshold temperature to change so that the near-field light is generated around the portion in which the index of refraction has changed. Since the recording layer is provided away from the mask layer by a distance that is not more than a distance which allows the near-field light to reach, the near-field light interacts with the recording layer and is scattered. The scattered light (propagation light) is partially reflected so as to generate the reflected light. The polarized component of the reflected light is detected by the reproduction means, thereby reproducing the information that has been recorded. Instead of the reflected light, the transmitted light may be detected to reproduce the information. During the reproduction, it does not occur that the magnetic field from the magnetic field generating means is applied to the optical recording medium.
Meanwhile, in the case where the magnetic layer is a magneto-optical recording layer, when the temperature rise occurs in the magneto-optical recording medium due to the temperature rise means, a portion, in the recording layer, whose coercive force becomes substantially zero occurs. When the magnetic field is applied to such a portion by the magnetic field generating means, the magnetization of such a portion is reversed, thereby carrying out the recording.
In contrast, the following procedure is carried out during the reproduction. More specifically, the temperature rise means is controlled by the temperature control means so that the temperature rise during the reproduction is lower than that of the recording. During this, the magnetization of the recording layer is not affected by the temperature rise. The temperature rise causes the index of refraction of the portion whose temperature is not less than the threshold temperature to change so that the near-field light is generated around the portion in which the index of refraction has changed. Since the recording layer is provided away from the mask layer by a distance that is not more than a distance which allows the near-field light to reach, the near-field light interacts with the recording layer and is scattered. The scattered light (propagation light) is partially reflected so as to generate the reflected light. The reproduction of the information is carried out in accordance with the reflected light. During the reproduction, it does not occur that the magnetic field from the magnetic field generating means is applied to the magneto-optical recording medium.
Accordingly, with the optical recording and reproduction apparatus, even when the distance between the mask layer and the recording layer is short so as to cause the thermal interference, the information that has been recorded is not affected. This is because, during the reproduction, the magnetization of the recording layer is not affected by the temperature rise and the magnetic field from the magnetic field generating means is not applied to the magneto-optical recording medium. Namely, even when the temperature rise occurs in the recording layer during the reproduction, since the magnetization of the recording layer is not affected by the temperature rise and the magnetic field is not applied to the magnetic layer, there is no reverse of magnetization in the recorded portion and the portions other than the recorded portion. This ensures to overcome the conventional deficiency that the information that has been recorded is gradually erased.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention.